Steel sheet for hot forming, hot-formed member, and method for manufacturing same

ABSTRACT

Disclosed are a high-strength and non-plated steel sheet which is for hot forming and may be suitable for use in automotive structural members that require collision resistance characteristics, a hot-formed member, and a method for manufacturing same. 
     A steel sheet for hot forming and a hot-formed member according to an embodiment of the present disclosure contain, in percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% of nitrogen (N), and 0.03 to 1.0% of niobium (Nb), with the remainder comprising iron (Fe) and inevitable impurities.

TECHNICAL FIELD

The present disclosure relates to a steel sheet for hot forming, ahot-formed member using the same, and a method for manufacturing thesame, and more particularly, to a high-strength and non-plated steelsheet which is for hot forming and may be suitable for use in automotivestructural members that require collision resistance characteristics, ahot-formed member, and a method for manufacturing the same.

BACKGROUND ART

As various safety regulations for protecting passengers of vehicles havebeen strengthened and interest in environmental issues has grownrecently, regulations on fuel efficiency and CO₂ emission have also beenstrengthened.

Accordingly, thickness of materials used for the vehicles may be reducedto increase fuel efficiency, but a decrease in thickness may cause astability problem in the vehicles and thus enhancement of strength ofthe material should be accompanied therewith.

A process of increasing strength of a material causes a decrease inelongation together with an increase in yield strength, resulting indeterioration of formability in most cases. Therefore, advanced highstrength steels (AHSS) such as dual phase (DP) steels and TRIP steelshave been developed based on studies on various materials and have beenactually applied to automobile parts, and such steel sheets exhibitexcellent formability compared to conventional high-strength steels forvehicles.

However, a higher forming force is required to form automobile partswith an increased strength of a material as described above, and thuscapacity and load of a press need to be increased. Also, molding of thehigh-strength material may cause a decrease in mold life and a decreasein productivity.

Although a martensitic steel having an ultra-high strength of 1,000 MPaor more may be effective on reducing a weight of the body of a vehiclewhen used in the vehicle, commercialization of the martensitic structureis difficult due to poor formability.

As methods for commercialization using a martensitic steel, a method ofpreparing a high-strength martensitic structure by cold forming aninitial ferritic structure having excellent formability, forming anaustenite by heat treatment at a high temperature, and quenching theresultant has been used. However, a problem of poor shape fixability mayoccur according to the above-described forming method due to phasetransformation in a non-constrained sate. Particularly, a volume changeis accompanied by a change in the crystal structure from FCC to BCT inphase transformation from austenite to martensite occurring during acooling process, and accordingly dimensional precision deteriorates.Thus, an additional process of performing dimensional correction isrequired.

To solve these problems, a forming method called hot press forming (HPF)or hot forming has recently been proposed. Hot press forming is aforming method to increase a strength of a final product by preparing anaustenite single phase by heating a steel sheet at a high temperaturehigher than Ac1 at which processing is easily performed, hot forming thesteel sheet by press forming, and forming a low-temperature structuresuch as martensite by quenching. The hot forming is advantageous in thata problem in formability caused during preparation of a high-strengthmaterial may be minimized.

However, because the steel sheet is heated to a high temperature in thecase of using the hot press forming method, the surface of the steelsheet is oxidized, and thus a process of removing oxides from thesurface of the steel sheet needs to be added after the press forming.

To solve these problems, a method disclosed in Patent Document 1 hasbeen proposed. In Patent Document 1, although a steel sheet coated withAl—Si is heated at a temperature of 850° C. or higher and thenhot-pressed to form a martensite structure, the steel sheet is notoxidized during heating due to an Al—coating layer formed on the surfaceof the steel sheet. When hot press forming is performed using theAl-coated steel sheet, not only a product having an ultra-high strengthof 1,000 MPa may be easily obtained but also a product having highdimensional precision may be obtained, and thus the hot press forminghas drawn attention and interest as a very effective method for formingautomobile parts on a decrease in weight and an increase in rigidity ofvehicles.

However, several problems have recently be raised in the hot pressforming method using an Al-coated steel sheet during a forming processand a subsequent bonding/welding process between other members.

Among them, according to Patent Document 2, because a plating layerincludes aluminum as a main phase, aluminum may be liquified at atemperature higher than a melting point of the plating layer to be fusedto a roll in a heating furnace when a blank is heated in a heatingfurnace or partial exfoliation may occur due to stress.

Also, according to Patent Document 3, a hot-pressed, formed member maybe prepared by bonding two or more members using an adhesive. In thiscase, a sufficient adhesive strength needs to be maintained to verifyadhesive strength. A method of testing whether the bonded portion iseasily maintained at a high strength by applying a tensile stress in adirection perpendicular to the bonded surface is often used. In thiscase, a plating layer is often detached from the inside of the platinglayer or an interface between the plating layer and a steel sheet. Inthis case, a problem of separation of the two members may occur evenunder a low stress.

In addition, according to Patent Document 4, a tailored welded blank(TWB), which is made by pre-bonding different steel sheets havingdifferent thicknesses for decreasing a weight of a vehicle, has beenused as a major material in hot press forming. The TWB is mainlyprepared by laser bonding and it is known that combination of thesurface condition of a material and strength of the raw materialconsiderably affects properties. However, in the case of a hot dip Alplated steel sheet, breakage of a welded part was observed when deformedby press forming after heat treatment. This is because Al on the platinglayer of the surface penetrates into the welded part during laserwelding of a TWB material and thus a ferrite phase remains in the weldedpart after heat treatment to embrittle the welded part. To overcomethis, an additional process of removing a surface film is suggestedbefore laser welding of the hot dip Al plated steel sheet.

As described above, aluminum plating is essential in order to preventoxidation during heating for hot press forming of a martensitic steel,but there is a need to develop technologies for solving various problemsoccurring thereby.

[Patent Document 1] US Patent Publication No. 6,296,805 (Oct. 2, 2001)

[Patent Document 2] Korean Patent Publication No. 10-1696121 (Jan. 6,2017)

[Patent Document 3] Korean Patent Application Publication No.10-2018-0131943 (Dec. 11, 2018)

[Patent Document 4] Korean Patent Application Publication No.10-2015-0075277 (Jul. 3, 2015)

DISCLOSURE Technical Problem

Embodiments of the present disclosure have been proposed to solveproblems described above and provided are a steel sheet for hot forminghaving ultra-high strength while preventing surface oxidation during hotpress forming without using a plating layer, a hot-formed member, and amethod for manufacturing the same.

Technical Solution

In accordance with an aspect of the present disclosure, a steel sheetfor hot forming includes, in percent by weight (wt %), 0.05 to 0.3% ofcarbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn),3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% ofnitrogen (N), 0.03 to 1.0% of niobium (Nb), and the remainder of iron(Fe) and inevitable impurities, wherein a microstructure comprises aferrite phase and 20 vol % or less of a carbonitride.

Also, according to an embodiment of the present disclosure, tThe ferritephase may have an average grain size of 100 μm or less.

Also, according to an embodiment of the present disclosure, the steelsheet may satisfy Expression (1) below:

0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0

Also, according to an embodiment of the present disclosure, a content ofCr may be from 3.5 to 5.5%.

Also, according to an embodiment of the present disclosure, the steelsheet may further include less than 3.0% of nickel (Ni).

Also, according to an embodiment of the present disclosure, the steelsheet may further include less than 0.1% of phosphorus (P) and less than0.01% of sulfur (S).

In accordance with another aspect of the present disclosure, ahot-formed member includes, in percent by weight (wt %), 0.05 to 0.3% ofcarbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn),3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% ofnitrogen (N), 0.03 to 1.0% of niobium (Nb), and the remainder of iron(Fe) and inevitable impurities.

Also, according to an embodiment of the present disclosure, thehot-formed member may satisfy Expression (1) below:

0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0

Also, according to an embodiment of the present disclosure, an averageoxygen content may be 20 wt % or less at a point of 0.1 μm depth fromthe surface.

Also, according to an embodiment of the present disclosure, thehot-formed member may have a yield strength of 1,100 MPa or more and atensile strength of 1,500 MPa or more.

Also, according to an embodiment of the present disclosure, a content ofCr may be from 3.5 to 5.5%.

Also, according to an embodiment of the present disclosure, thehot-formed member may further include less than 3.0% of nickel (Ni).

Also, according to an embodiment of the present disclosure, thehot-formed member may further include less than 0.1% of phosphorus (P)and less than 0.01% of sulfur (S).

In accordance with another aspect of the present disclosure, a methodfor manufacturing a hot-formed member includes: preparing a steel sheetfor hot forming comprising, in percent by weight (wt %), 0.05 to 0.3% ofcarbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn),3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% ofnitrogen (N), 0.03 to 1.0% of niobium (Nb), and the remainder of iron(Fe) and inevitable impurities; heating the steel sheet at a rate of 1to 1,000 ° C./sec to a temperature range of Ac3+50° C. to Ac3+200° C.and maintaining for 1 to 1,000 seconds; and hot-forming the heated andmaintained steel sheet and cooling the steel sheet at a rate of 1 to1000° C./sec to a temperature below Mf

Also, according to an embodiment of the present disclosure, the steelsheet for hot forming may satisfy Expression (1) below.

0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1)

Also, according to an embodiment of the present disclosure, the steelsheet for hot forming may include a microstructure comprising a ferritephase and 20 vol % or less of a carbonitride, wherein an average grainsize of the ferrite phase is 100 μm or less.

Also, according to an embodiment of the present disclosure, a content ofCr in the steel sheet for hot forming may be from 3.5 to 5.5%.

Also, according to an embodiment of the present disclosure, the steelsheet for hot forming may further include less than 3.0% of nickel (Ni).

Also, according to an embodiment of the present disclosure, the steelsheet for hot forming may further include less than 0.1% of phosphorus(P) and less than 0.01% of sulfur (S).

Also, according to an embodiment of the present disclosure, thepreparing of the steel sheet for hot forming may include: reheating aslab in a temperature range of 1,000 to 1,300° C.; preparing ahot-rolled steel sheet by finish-rolling the reheated slab in atemperature range higher than Ar3 and equal to or lower than 1,000° C.;coiling the hot-rolled steel sheet in a temperature range higher than Msand equal to or lower than 850° C.; and acid-pickling the coiled,hot-rolled steel sheet.

Also, according to an embodiment of the present disclosure, the methodmay further include: preparing a cold-rolled steel sheet by rolling theacid pickled, hot-rolled steel sheet with a reduction ratio of 30 to80%; and continuously annealing the cold-rolled steel sheet in atemperature range of 700 to 900° C.

Also, according to an embodiment of the present disclosure, the methodmay further include batch-annealing the coiled, hot-rolled oracid-pickled steel sheet in a temperature range of 500 to 850° C. for 1to 100 hours.

Advantageous Effects

In the steel sheet for hot forming and the hot-formed member accordingto an embodiment of the present disclosure, surface oxidation isprevented during hot press forming by improving oxidation resistance bycontrolling alloying elements, and thus conventional aluminum platingmay be omitted.

In addition, problems, which may occur during a hot press formingprocess and a bonding/welding process performed between differentmembers when an Al-coated steel sheet is used, may be solved.

In addition, high strength at an equivalent level to that ofconventional Al-plated steel materials may be obtained.

DESCRIPTION OF DRAWINGS

FIG. 1 is an electron microscope image showing a microstructure of asteel sheet for hot forming according to an embodiment of the presentdisclosure.

FIG. 2 is a photograph exemplarily illustrating good formability (a) andpoor formability (b) obtained when hot forming is performed using amini-bumper mold.

FIG. 3 is a graph illustrating tensile test results of samples ofexamples and comparative examples which are hot-formed using aplate-shaped mold.

FIGS. 4 and 5 are electron microscope images of microstructures of steelsheets for hot forming according to an example and a comparative exampleprior to formation, respectively.

FIGS. 6 and 7 are graphs illustrating GDS analysis results of hot-formedmembers obtained using a mini-bumper mold according to an exampleexhibiting good oxidation resistance and a comparative exampleexhibiting inferior oxidation resistance with respect to depth from thesurface.

BEST MODE

A steel sheet for hot forming according to an embodiment of the presentdisclosure may include, in percent by weight (wt %), 0.05 to 0.3% ofcarbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn),3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% ofnitrogen (N), 0.03 to 1.0% of niobium (Nb), and the remainder of iron(Fe) and inevitable impurities, wherein a microstructure includes aferrite phase and 20 vol % or less of a carbonitride.

MODES OF THE INVENTION

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. The followingembodiments are provided to fully convey the spirit of the presentdisclosure to a person having ordinary skill in the art to which thepresent disclosure belongs. The present disclosure is not limited to theembodiments shown herein but may be embodied in other forms. In thedrawings, parts unrelated to the descriptions are omitted for cleardescription of the disclosure and sizes of elements may be exaggeratedfor clarity.

All of the above-described problems occurring during the hot formingprocess and the bonding/welding process are caused by presence of aplating layer. The present inventors have designed optimum alloyingelements such as Cr, Si, and Mn to obtain high strength at an equivalentlevel to that of conventional plated-steel sheet, to inhibit surfaceoxidation without using a plated layer, and to have excellentformability suitable for preparation of a formed member.

A steel sheet for hot forming and a hot-formed member according to anembodiment of the present disclosure may include, in percent by weight(wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% andless than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and theremainder of iron (Fe) and inevitable impurities.

Hereinafter, reasons for numerical limitations on the contents ofalloying elements in the embodiment of the present disclosure will bedescribed. Hereinafter, the unit is wt % unless otherwise stated.

The content of C is from 0.05 to 0.3%.

C is an element not only effective on stabilization of an austenitephase but also effective on obtaining high strength by solid solutionstrengthening effects. However, an excess of C may not only deteriorateprocessibilty due to an increase in a carbide in a microstructure butalso deteriorate physical and mechanical properties (e.g., ductility,toughness, and corrosion resistance) of a welded part and aheat-affected portion. Therefore, an upper limit thereof is set to 0.3%.In addition, as described above, C needs to be added in an amount of0.05% or more to obtain stability of the austenite stability and targetmechanical properties. Preferably, C may be added in an amount of 0.15%or more to obtain high strength. However, because high strength may becomplemented by adding N and formation of a Cr carbide deterioratesoxidation resistance, the C content is not necessarily 0.15% or more.

The content of Si is from 0.5 to 3.0%.

Si, serving as a deoxidizer during a steelmaking process, is effectiveon enhancing corrosion resistance and oxidation resistance, and theseproperties are effective when the Si content is 0.5% or more. However,because Si is an element effective on stabilizing a ferrite phase, anexcess of Si may promote formation of delta (δ) ferrite in a cast slab,thereby not only deteriorating hot processibility but also deterioratingductility and toughness of a steel material due to solid solutionstrengthening effects. Therefore, an upper limit thereof is set to 0.7%.Preferably, Si may be added in an amount of 1.0 to 2.0%.

The content of Mn is from 0.1 to 2.0%.

Mn, as an element effective on stabilizing an austenite phase, isessential to obtain the austenite phase at a high temperature duringheat treatment and is added in an amount of 0.1% or more. However, anexcess of Mn not only causes an increases in S-based inclusions (MnS)leading to deterioration of ductility, toughness, and corrosionresistance of a steel material but also deteriorates oxidationresistance due to an increase in MnO on the surface of the steelmaterial during heat treatment at a high temperature in an oxidizingatmosphere for forming an austenite structure. Therefore, an upper limitthereof is set to 2.0%.

The content of Cr is from 3.0 to 9.0%.

Cr, as a ferrite-stabilizing element, is effective on improvingcorrosion resistance and oxidation resistance, and these properties areeffective when the Cr content is 3.0% or more. However, an excess of Crmay cause an increase in Ac1 due to enhancement of stability of ferritemaking it difficult to obtain an austenite phase during heat treatmentof a steel material. Therefore, an upper limit thereof is set to 9.0%.In consideration of hot formability and economic efficiency, the Crcontent may be from 3.5 to 7.0%, preferably, from 3.5 to 5.5%.

The content of N is more than 0% and less than 0.2%.

N, as not only an austenite phase-stabilizing element but also anelement effective on obtaining high strength by solid solutionstrengthening effects, may decrease the amounts of Ni and Mn, therebypreventing an increase in costs of materials. However, an excess of Nmay cause formation of a large amount of a nitride in a microstructure,thereby deteriorating processibility. Also, when more than a certainlevel of N is added, delta (δ) ferrite formed during a cooling processafter casting may cause local formation of nitrogen pin holes, therebydeteriorating quality. Therefore, an upper limit thereof is set to 0.2%.

The content of Nb is from 0.03 to 1.0%.

Nb, forming a carbonitride of Nb(C,N) at a high temperature, iseffective on preventing coarsening of grains during heat treatment andthis property is effective when the Nb content is 0.03% or more. Suchgrain refinement is effective not only on improving processibilty of asteel material at a high temperature but also on enhancing impactresistance. However, an excess of Nb may cause formation of a largeamount of the Nb(C,N) carbonitride, thereby decreasing amounts of soluteC and N, making it difficult to obtain target mechanical properties.Therefore, an upper limit thereof is set to 1.0%, preferably 0.3%.

The content of Ni is less than 3.0%.

Although used as a strong austenite phase-stabilizing element, Ni is notan essential element in the present disclosure because manufacturingcosts are increased thereby. However, when Ni is added within an upperlimit of 3.0%, an austenite phase may be easily formed at a hightemperature. However, when the Ni content is 3.0% or more, residualaustenite is excessively formed in a cooled structure after heattreatment and thus strength may deteriorate. Therefore, an upper limitthereof is set to 3.0%.

The content of P is less than 0.1%.

Because P deteriorates corrosion resistance or hot processibilty, anupper limit thereof is set to 0.1%.

The content of S is less than 0.01%.

Because S deteriorates corrosion resistance or hot processibilty, anupper limit thereof is set to 0.01%.

The remaining component of the composition of the present disclosure isiron (Fe). However, the composition may include unintended impuritiesinevitably incorporated from raw materials or surrounding environments,and thus addition of other alloy components is not excluded. Theimpurities are not specifically mentioned in the present disclosure, asthey are known to any person skilled in the art of manufacturing.

The steel sheet for hot forming of the present disclosure has amicrostructure including a ferrite phase and 20 vol % or less of acarbonitride. Because good hot formability is required to prevent cracksor bursts on the surface during hot forming, e.g., hot press forming(HPF), grain refinement is required in a ferrite phase.

The steel sheet for hot forming according to an embodiment of thepresent disclosure may include a ferrite phase having an average grainsize of 100 μm or less. In the present disclosure, the average grainsize of the ferrite phase is controlled by the chemical composition ofalloying elements. By adding Nb as described above, a carbonitride isformed to reduce in size of grains and coarsening of grains may beprevented at a high temperature, and thus addition of Nb is essential.The ranges of contents of C and N which form the carbonitride with Nbare also important to control the average grain size. When the contentof Cr is too low, e.g., less than 3.0%, grains are coarsened, therebydeteriorating formability. As will be descried below, the steel sheetfor hot forming may be a hot-rolled steel sheet obtained by batchannealing, a cold-rolled steel sheet obtained by continuous annealing,or a hot-rolled steel sheet obtained by acid pickling without performingannealing. Although the grain size of steel sheets provided to hotforming may generally be controlled by annealing, excellent formabilitymay be obtained during hot forming regardless of performing annealingwhen the range of the chemical composition of alloying elements of thepresent disclosure is satisfied.

In addition, according to an embodiment of the present disclosure, thesteel sheet for hot forming may satisfy Expression (1) below.

0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1)

According to the present disclosure, excellent oxidation resistance maybe obtained by adjusting the contents of Si, Cr, C, and Mn to satisfyExpression (1) although a plating layer is not formed. Although thecontents of oxidation-inhibiting elements such as Cr and Si have thegreatest influence on oxidation resistance of a hot-formed member, theoxidation resistance is also sensitive to the contents of C and Mn thatpromote formation of precipitates and oxides as well thereby derivingExpression (1) above. When the contents of Cr and Si are low, denseformation of Cr and Si oxides is inhibited and a thick Fe oxide isformed on the surface layer. In addition, when a large amount of C isadded, formation of a Cr carbide increases to reduce the Cr content in amatrix, thereby causing formation of an Fe oxide. In addition, when alarge amount of Mn is locally added, a Mn oxide is formed therebydeteriorating oxidation resistance on the surface.

Oxidation behavior of the surface layer sensitively changes during hotforming due to influence of various alloying elements as describedabove, It is important to define the quality of oxidation resistance ofthe surface layer, and the hot-formed member according to an embodimentof the present disclosure may have an average oxygen content of 20 wt %or less at a point of 0.1 μm depth from the surface.

Then, a method of preparing a steel sheet for hot forming and ahot-formed member will be described.

First, a steel sheet for hot forming may be manufactured according to awell-known manufacturing process as a cold-rolled steel sheet or anacid-pickled, hot-rolled steel sheet, but manufacturing conditions arenot particularly limited. An example of the method for manufacturing thesteel sheet for hot forming is as follows.

An ingot or slab having the above-described chemical composition ofalloying elements is heated in a temperature range of 1,000 to 1,300° C.and hot-rolled. At a heating temperature below 1,000° C., it isdifficult to homogenize the slab structure, and at a heating temperatureexceeding 1,300° C., an oxide layer may be excessively formed andmanufacturing costs may increase.

Subsequently, hot finish rolling is performed in a temperature rangehigher than Ar3 and equal to or lower than 1,000° C. At a finish rollingtemperature of Ar3 or less, recrystallization rolling may be easilyinduced making it difficult to control formation of a surface mixedstructure and a steel sheet. When the finish rolling temperature exceeds1,000° C., hot-rolled grains may be easily coarsened.

The hot-rolled steel sheet may be coiled in a temperature range higherthan Ms and equal to or lower than 850° C. When a coiling temperature isMs or below, it is difficult to perform a subsequent cold rolling due totoo high strength of the hot-rolled steel. When the coiling temperatureis higher than 850° C., a thickness of an oxide layer excessivelyincreases making it difficult to perform acid pickling on the surface.

The hot-rolled steel sheet may be hot-formed immediately after acidpickling. Meanwhile, the acid pickling and cold rolling may be performedto control the thickness of the steel sheet more precisely. Although acold rolling reduction ratio after acid pickling is not particularlylimited, the cold rolling may be performed with a reduction ratio of 30to 80% to obtain a target thickness. In this regard, to reduce a rollingload of the cold rolling, if required, the hot-rolled steel sheet or thepreviously acid-pickled, hot-rolled steel sheet may be batch-annealed.In this regard, although batch annealing conditions are not particularlylimited, the batch annealing may be performed at a temperature of 500 to850° C. for 1 to 100 hours to reduce strength of the hot-rolled steelsheet.

The cold-annealed, cold-rolled steel sheet may be continuously annealed.Although a continuous annealing heat treatment process is notparticularly limited, the heat treatment may be performed in atemperature range of 700 to 900° C.

Subsequently, the hot-rolled steel sheet or cold-rolled, annealed steelsheet prepared as described above may be hot-formed to prepare ahot-formed member.

The prepared steel sheet for hot forming is heated to a temperaturerange of Ac3+50° C. to Ac3+200° C. at a heating rate of 1 to 1,000°C./sec. At a heating rate below 1° C./sec, it is difficult to obtainsufficient productivity. Also, a too long heating time not onlyexcessively increases a grain size to deteriorate impact toughness butalso excessively forms oxides on the surface of the formed member todeteriorate spot weldability. To increase the heating rate to exceed1,000° C./sec, expensive equipment is required.

Subsequently, the heat treatment may be maintained in the temperaturerange of Ac3+50° C. to Ac3+200° C. for 1 to 1,000 seconds. At a heatingtemperature below Ac3+50° C., there is a high possibility that ferriteis formed while a blank is transferred from a heating furnace to a mold,thereby failing to obtain a target strength. When the heatingtemperature exceeds Ac3+200° C., an excess of oxides on the surface ofthe formed member makes it difficult to obtain spot weldability andcoating property during a subsequent process.

The hot-formed member is cooled to a temperature below Mf simultaneouslywith the hot forming and a cooling rate may be controlled in a range of1 to 1000° C./sec. At a cooling rate below 1° C./sec, undesirableferrite is formed making it difficult to obtain a tensile strength 1,500MPa or more. On the contrary, to obtain a cooling rate exceeding 1,000°C./sec, expensive, specified equipment is required.

Hereinafter, the present disclosure will be described in more detainwith reference to the following examples.

EXAMPLES

Ingot materials having chemical compositions of alloying elements shownin Table 1 were below melted, heated in a furnace at a temperature of1,180° C. for 2 hours, and hot-rolled to obtain hot-rolled steel sheetshaving a final thickness of 3 mm. Subsequently, the hot-rolled steelsheets were acid-pickled for cold rolling, cold-rolled with a reductionratio of 60%, and annealed at 760° C. to obtain steel sheets for hotforming.

TABLE 1 Steel type (wt %) C Si Mn P S Cr Ni N Nb Others ComparativeExample 1 0.219 1.47 0.5 0.012 0.002 5.0 0.2 0.019 0 Comparative Example2 0.222 1.51 0.5 0.014 0.004 3.97 0.196 0.016 0 Comparative Example 30.22 1.51 1.48 0.018 0.002 4.0 0.198 0.02 0 Comparative Example 4 0.2151.99 1.5 0.016 0.003 4.0 0.201 0.016 0 Comparative Example 5 0.217 2.451.48 0.012 0.002 3.98 0.197 0.016 0 Comparative Example 6 0.223 1.551.51 0.012 0.003 3.93 0.203 0.018 0 Al: 0.51 Comparative Example 7 0.2251.49 1.48 0.014 0.004 3.93 201 0.02 0 Al: 1.02 Comparative Example 80.223 1.49 0.5 0.016 0.002 7.05 0.198 0.021 0 Comparative Example 90.225 1.48 1.47 0.017 0.002 6.93 0.2 0.027 0 Comparative Example 10 0.140.4 0.48 0.016 0.003 11.3 0.39 0.05 0.16 B: 0.0038 Comparative Example11 0.179 1.5 0.52 0.013 0.002 3.98 0.2 0.027 0 Comparative Example 120.182 1.5 0.5 0.014 0.004 4.0 0.2 0.028 0 B: 0.0054 Comparative Example13 0.135 1.47 0.49 0.012 0.003 3.87 0.2 0.027 0 Comparative Example 140.14 1.5 0.49 0.018 0.003 4.04 0.2 0.03 0 B: 0.0038 Comparative Example15 0.139 1.51 0.51 0.014 0.002 4.0 0.2 0.029 0 B: 0.0083 ComparativeExample 16 0.265 1.49 0.498 0.016 0.002 3.97 0.203 0.031 0 ComparativeExample 17 0.295 1.49 0.492 0.018 0.002 4.05 0.206 0.033 0 ComparativeExample 18 0.216 1.5 0.512 0.014 0.002 4.0 0.2 0.031 0.096 Sb: 0.043Comparative Example 19 0.202 1.49 0.495 0.013 0.002 3.88 0.196 0.0260.103 Sb: 0.046 Comparative Example 20 0.25 1.53 0.512 0.014 0.002 4.990.201 0.026 0.101 Sb: 0.055 Comparative Example 21 0.225 1.5 0.506 0.0160.004 2.97 0.212 0.028 0.098 Sb: 0.048 Comparative Example 22 0.216 1.510.495 0.012 0.004 1.92 0.204 0.028 0.101 Sb: 0.05 Comparative Example 230.258 1.5 0.496 0.012 0.003 2.94 0.204 0.03 0.047 Sb: 045 Example 10.215 1.49 0.495 0.013 0.002 3.96 0.203 0.032 0.095 Example 2 0.217 1.490.496 0.016 0.004 4.99 0.196 0.031 0.1 Example 3 0.215 1.49 0.493 0.0120.002 4.49 0.197 0.035 0.07 Example 4 0.238 1.5 0.505 0.016 0.004 5.00.2 0.028 0.102 Example 5 0.242 1.52 0.497 0.011 0.003 5.02 0.201 0.0270.105 Sn: 0.052 Example 6 0.234 1.64 0.61 0.016 0.004 4.61 0.28 0.0210.096 Al: 1.12 Example 7 0.217 1.5 0.496 0.012 0.003 4.0 0.206 0.0310.052

FIG. 1 is an electron microscope image illustrating a microstructure ofa steel sheet for hot forming according to an embodiment of the presentdisclosure. Referring to FIG. 1, it may be confirmed that amicrostructure of a cold-rolled, annealed steel sheet for hot formingincludes 20 vol % of a carbonitride in a ferrite matrix structure.

The steel sheets for hot forming prepared as described above werehot-formed and heat treatment conditions therefor are shown in Table 2below. The steel sheets were put into a furnace pre-heated to 950° C.,maintained for 5.5 minutes, air-cooled for 12 seconds, hot-formed in amold, and quenched to room temperature at a cooling rate of 30° C./secor more.

Two types of molds were used to form the hot-formed member. A first moldwas a plate-shaped mold for forming the hot-formed member and performinga tensile test to evaluate physical properties after hot forming, and asecond mold was prepared as a mini-bumper mold to evaluate formabilityand oxidation resistance.

Samples of the formed members obtained using the plate-shaped mold wereevaluated by the tensile test according to the JIS 13 B standards andthe results are shown in Table 2. In addition, formability and oxidationresistance of the formed members obtained by using the mini-bumper moldwere evaluated by applying the same hot forming heat treatmentconditions and the results are shown in Table 2.

FIG. 2 is a photograph exemplarily illustrating good formability (a) andpoor formability (b) obtained when hot forming is performed using amini-bumper mold after hot forming. As shown in (b) of FIG. 2, cracks orbursts occurred on the surfaces during hot forming in some of thecomparative examples and they were indicated as “poor” in Table 2. Onthe contrary, good formability as shown in (a) of FIG. 2 was indicatedas “good”.

Oxidation resistance of the hot-formed members obtained using themini-bumper mold was evaluated based on whether excessive oxide scaleswere locally formed on the surface. A case in which surface oxidationwas inhibited was indicated as “good” and a case in which excessiveoxide scales were locally formed was indicated as “inferior”.

TABLE 2 Heat treatment conditions Tensile test properties Properties ofhot-formed for hot forming Yield Tensile member Temperature Timestrength strength Elongation Expression Oxidation Example Atmosphere (°C.) (min) (MPa) (MPa) (%) Formability (1) resistance Comparative Example1 air 950 5.5 1,075 1,564 7.7 poor 0.628 good Comparative Example 2 air950 5.5 1,029 1,517 8.2 poor 0.062 good Comparative Example 3 air 9505.5 1,107 1,643 7.6 poor −1.335 inferior Comparative Example 4 air 9505.5 1,176 1,744 7.1 poor −0.962 inferior Comparative Example 5 air 9505.5 1,202 1,814 7.5 poor −0.583 inferior Comparative Example 6 air 9505.5 1,108 1,605 6.8 poor −1.397 good Comparative Example 7 air 950 5.5969 1,491 8.9 poor −1.408 good Comparative Example 8 air 950 5.5 1,1411,644 6.8 poor 1.798 good Comparative Example 9 air 950 5.5 1,180 1,7317.2 poor 0.308 good Comparative Example 10 air 950 5.5 1,086 1,411 8.5poor 3.671 good Comparative Example 11 air 950 5.5 995 1,411 8.1 poor0.183 good Comparative Example 12 air 950 5.5 979 1,405 9.2 poor 0.213good Comparative Example 13 air 950 5.5 905 1,304 9.9 poor 0.295 goodComparative Example 14 air 950 5.5 920 1,303 9 poor 0.398 goodComparative Example 15 air 950 5.5 897 1,286 8.8 poor 0.358 goodComparative Example 16 air 950 5.5 1,206 1,723 7.2 good −0.103 inferiorComparative Example 17 air 950 5.5 1,256 1,804 7.3 good −0.154 inferiorComparative Example 18 air 950 5.5 1,180 1,657 8.2 good 0.075 inferiorComparative Example 19 air 950 5.5 1,411 1,796 10.2 good 0.073 inferiorComparative Example 20 air 950 5.5 1,189 1,704 7.4 good 0.543 inferiorComparative Example 21 air 950 5.5 1,150 1,645 9.1 poor −0.535 inferiorComparative Example 22 air 950 5.5 1,089 1,599 9.3 poor −1.078 inferiorComparative Example 23 air 950 5.5 1,187 1,701 8.6 poor −0.654 inferiorExample 1 air 950 5.5 1,140 1,596 8.8 good 0.073 good Example 2 air 9505.5 1,127 1,597 7.6 good 0.651 good Example 3 air 950 5.5 1,135 1,5978.2 good 0.378 good Example 4 air 950 5.5 1,165 1,662 7.6 good 0.578good Example 5 air 950 5.5 1,174 1,679 7.6 good 0.602 good Example 6 air950 5.5 1,203 1,735 7.8 good 0.329 good Example 7 air 950 5.5 1,1101,553 8.5 good 0.095 good

FIG. 3 is a graph illustrating tensile test results of the hot-formedsamples of examples and comparative examples using a plate-shaped mold,and the tensile test was performed according to JIS 13 B standards. Uponcomparison among all of the tensile test curves of the examples andcomparative examples, it was confirmed that fracture did not occurbefore exhibiting a maximum strength but occurred after the maximumtensile strength was obtained as shown in FIG. 3.

With regard the results, to evaluate hydrogen delayed fractureresistance of an Al-plated hot-formed member, a method of measuring theH content in a steel sheet has been known. According to Patent Document2 (Korean Patent Publication No. 10-1696121), occurrence of a fracturewas observed before a maximum strength was obtained in a tensile curve,and a normal fracture was not observed in the tensile test due to thehigh H content in the steel sheet. That is, this indicates that hydrogendelayed fracture resistance may be judged based on the results of thetensile curve obtained from the tensile test. In the case of thehot-formed member prepared using the chemical composition of thealloying elements according to the present disclosure, a tensilebehavior, in which fracture occurred after a tensile strength reached amaximum level, was observed and thus excellent hydrogen delayed fractureresistance was confirmed.

Upon evaluation of formability of the hot-formed members shown in Table2, grain size of the steel sheets for hot forming was confirmed as afactor the most significantly affecting the formability. That is, inmost cases in which formability of steel types indicated as “poor” inTable 2, the C content was low or the grain refining element such as Nbwas not added, and this result was more clearly identified by observinga microstructure thereof. FIGS. 4 and 5 are electron microscope imagesof microstructures of steel sheets for hot forming according to anexample and a comparative example prior to formation, respectively. FIG.4 is a photograph of the microstructure of Example 2 before hot forming,and FIG. 5 is a photograph of the microstructure of Comparative Example1 before hot forming. It was confirmed that the steel types havingformability indicated by “poor” had a coarse ferrite grain size of 100μm or more before hot forming as shown in FIG. 5. Based on theseresults, it was confirmed that the average grain size of ferrite in themicrostructure needs to be controlled to 100 μm or less to obtain goodformability in the final hot-formed member.

Meanwhile, it was confirmed that excellent oxidation resistance of thehot-formed member was obtained when the contents of Cr and Si which areoxidation-suppressing elements and the contents of C and Mn which areelements forming precipitates and oxides satisfy Expression (1) asdescribed above based on Table 2.

Oxidation resistance quality of surface layers during hot forming wereclassified into good and inferior by visual observation based on glowdischarge spectrometer (GDS) analysis results, and representativeresults are shown in FIGS. 6 and 7. FIGS. 6 and 7 are graphsillustrating GDS analysis results of hot-formed members using amini-bumper mold according to an example exhibiting good oxidationresistance and a comparative example exhibiting inferior oxidationresistance with respect to depth from the surface. As a result ofanalyzing contents of the alloying elements with respect to depth in thethickness direction from the surface by the GDS, a difference of oxygencontents between the hot-formed member having good oxidation resistanceand that having inferior oxidation resistance was clearly observed.While the average oxygen content exceeds 20 wt % at a point of 0.1 μmdepth from the surface in the comparative example exhibiting inferioroxidation resistance of FIG. 7, it was confirmed that the average oxygencontent was about 2 to 3 wt % at a point of 0.1 μm depth from thesurface in the example exhibiting good oxidation resistance of FIG. 6.Based on these results, it was confirmed that the average oxygen contentneeds to be controlled to 20 wt % or less at a point of 0.1 μm depthfrom the surface to obtain good oxidation resistance of a finalhot-formed member.

The comparative examples and examples of Table 2 will be described inmore detail below.

In Comparative Examples 1 to 9 to which Nb was not added, grainrefinement did not occur before hot forming, and thus poor formabilitywas obtained. Among them, inferior oxidation resistance was observed inComparative Examples 3 to 5 due to negative values of Expression (1).However, in the cases of Comparative Examples 6 and 7, good oxidationresistance was obtained despite negative values of Expression (1)because Al, effective on oxidation resistance, was added in an amount of0.5% or more.

In Comparative Example 10, poor formability was obtained despiteaddition of Nb due to the high Cr content and good oxidation resistancewas obtained despite the low Si content because Expression (1) wassatisfied by the high Cr content.

In Comparative Examples 10 to 15 where the C content was slightly loweven within the range proposed by the present disclosure, and thus itwas confirmed that the yield strength and the tensile strength did notreach 1,100 MPa and 1,500 Mpa, respectively. However, in ComparativeExample 10 where the N content was high as 0.05%, a result close to thetarget strength was obtained and thus it was confirmed that highstrength property may be complemented by adding N.

Good formability was obtained in Comparative Examples 16 and 17 althoughNb was not added and this was because oxidation resistance moredeteriorated by formation of a large amount of a carbide due to aslightly high C content but formability was improved due to oxidescales.

Sb was further added to the steel types of Comparative Examples 18 to23. Sb was oxidized at a hot forming temperature of 950° C. to bepresent as scales in the form of ash resulting in inferior oxidationresistance although Expression (1) was satisfied in Comparative Examples18 to 20.

In Comparative Examples 21 to 23, poor formability was obtained despiteaddition of Nb, and it was confirmed that this is because the grainscoarsened due to the low Cr content to deteriorate formability.

While the present disclosure has been particularly described withreference to exemplary embodiments, it should be understood by those ofskilled in the art that various changes in form and details may be madewithout departing from the spirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

The steel sheet for hot forming according to the present disclosure maybe applied to automotive structural members because ultra-high strengthmay be obtained simultaneously inhibiting surface oxidation during hotpress forming without using a plating layer.

1. A steel sheet for hot forming comprising, in percent by weight (wt%), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0%of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and lessthan 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and theremainder of iron (Fe) and inevitable impurities, wherein amicrostructure comprises a ferrite phase and 20 vol % or less of acarbonitride.
 2. The steel sheet according to claim 1, wherein theferrite phase has an average grain size of 100 μm or less.
 3. The steelsheet according to claim 1, wherein the steel sheet satisfies Expression(1) below:0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1) (wherein Si, Cr, C, and Mndenote contents (wt %) of the elements, respectively).
 4. The steelsheet according to claim 1, wherein a content of Cr is from 3.5 to 5.5%.5. The steel sheet according to claim 1, further comprising less than3.0% of nickel (Ni).
 6. The steel sheet according to claim 1, furthercomprising less than 0.1% of phosphorus (P) and less than 0.01% ofsulfur (S).
 7. A hot-formed member comprising, in percent by weight (wt%), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0%of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and lessthan 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and theremainder of iron (Fe) and inevitable impurities.
 8. The hot-formedmember according to claim 7, wherein the hot-formed member satisfiesExpression (1) below:0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1) (wherein Si, Cr, C, and Mndenote contents (wt %) of the elements, respectively).
 9. The hot-formedmember according to claim 7, wherein an average oxygen content is 20 wt% or less at a point of 0.1 μm depth from the surface.
 10. Thehot-formed member according to claim 7, wherein the hot-formed memberhas a yield strength of 1,100 MPa or more and a tensile strength of1,500 MPa or more.
 11. The hot-formed member according to claim 7,wherein a content of Cr is from 3.5 to 5.5%.
 12. The hot-formed memberaccording to claim 7, further comprising less than 3.0% of nickel (Ni).13. The hot-formed member according to claim 7, further comprising lessthan 0.1% of phosphorus (P) and less than 0.01% of sulfur (S).
 14. Amethod for manufacturing a hot-formed member, the method comprising:preparing a steel sheet for hot forming comprising, in percent by weight(wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% andless than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and theremainder of iron (Fe) and inevitable impurities; heating the steelsheet at a rate of 1 to 1,000° C./sec to a temperature range of Ac3+50°C. to Ac3+200° C. and maintaining for 1 to 1,000 seconds; andhot-forming the heated and maintained steel sheet and cooling the steelsheet at a rate of 1 to 1000° C./sec to a temperature below Mf.
 15. Themethod according to claim 14, wherein the steel sheet for hot formingsatisfies Expression (1) below.0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1) (wherein Si, Cr, C, and Mndenote contents (wt %) of the elements, respectively).
 16. The methodaccording to claim 14, wherein the steel sheet for hot forming comprisesa microstructure comprising a ferrite phase and 20 vol % or less of acarbonitride, wherein an average grain size of the ferrite phase is 100μm or less.
 17. The method according to claim 14, wherein a content ofCr in the steel sheet for hot forming is from 3.5 to 5.5%.
 18. Themethod according to claim 14, wherein the steel sheet for hot formingfurther comprises less than 3.0% of nickel (Ni).
 19. The methodaccording to claim 14, wherein the steel sheet for hot forming furthercomprises less than 0.1% of phosphorus (P) and less than 0.01% of sulfur(S).
 20. The method according to claim 14, wherein the preparing of thesteel sheet for hot forming comprises: reheating a slab in a temperaturerange of 1,000 to 1,300° C.; preparing a hot-rolled steel sheet byfinish-rolling the reheated slab in a temperature range higher than Ar3and equal to or lower than 1,000° C.; coiling the hot-rolled steel sheetin a temperature range higher than Ms and equal to or lower than 850°C.; and acid-pickling the coiled, hot-rolled steel sheet.
 21. The methodaccording to claim 20, further comprising: preparing a cold-rolled steelsheet by rolling the acid pickled, hot-rolled steel sheet with areduction ratio of 30 to 80%; and continuously annealing the cold-rolledsteel sheet in a temperature range of 700 to 900° C.
 22. The methodaccording to claim 20, further comprising batch-annealing the coiled,hot-rolled or acid-pickled steel sheet in a temperature range of 500 to850° C. for 1 to 100 hours.